August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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子程序
｜
77
了
Perl
“去除不必要限制”的理念。当然，这和许多传统程序语言不一样，它们习惯
于“强类型（
strictly typed
）”子程序，也就是只允许一定个数的参数并且预先限制
它们的类型。
Perl
本身的灵活是件好事，当以超出预期数量的参数调用子程序时（如
之前的子程序
&max
）
，也许会造成问题。
当然，我们可以通过检查
@_
数组的长度来判断参数个数是否符合预期。比如
将
&max
写成下面这样，先检查其参数列表：
sub max {
if (@_ != 2) {
print "WARNING! &max should get exactly two arguments!\n";
}
#
其余代码和前面一样……
}
上面的
if
判断是在标量上下文中直接使用数组“名称”来取得数组元素的个数，你
应该在第
3
章中见过这个用法。
但在实际编写的
Perl
程序中，这种检查方式很少见，更好的做法是让子程序自动适应
任意数目的参数。
改进版的
&max
子程序
现在让我们来改写
&max
，使它能够接受任意数目的参数：
$maximum = &max(3, 5, 10, 4, 6);
sub max {
my($max_so_far) = shift @_; #
数组中的第一个值，就是目前所见的最大值
foreach (@_) { #
遍历数组
@_
中的其他元素
if ($_ > $max_so_far) { #
当前元素比
$max_so_far
更大吗？
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.